The present invention relates in general to reproduction apparatus utilizing an electrically biased roller for transferring a marking particle image from an image bearing dielectric support member to a receiver member, and more particularly to control for the electrical bias of the reproduction apparatus transfer roller in order to optimize cleaning thereof, particularly during a start-up phase of reproduction.
In typical commercial electrostatographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photo-conductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern at a developing station to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle image transfer may be accomplished by ion emission from a corona charger onto the receiver member while in contact with the dielectric support member. Alternatively, an electrically biased roller, urging the receiver member against the dielectric support member, has been used to cause the marking particles on the dielectric support member to move to the receiver members. That is, the transfer roller is electrically biased so as to charge the receiver member with the opposite polarity to that of the marking particles. Roller transfer apparatus offer certain advantages over corona transfer apparatus in that the roller transfer apparatus substantially eliminate defects in the transferred image due to paper cockle or marking particle flakes. This result stems from the fact that the pressure of the roller urging the receiver member against the dielectric support member is remarkably efficient in providing intimate uniform contact therebetween.
However, during operation of roller transfer apparatus, background marking particles, or marking particles outside the area of the receiver member may be picked up by the transfer roller resulting in contamination of the roller. Transfer roller contamination may eventually result in contamination of the backside of receiver members passing between the transfer roller and the dielectric support member. The backside of the receiver members are those sides facing the transfer roller surface. In order to minimize transfer roller contamination, a cleaning subsystem may be added to the roller transfer assembly. The cleaning subsystem that is typically used in current practice includes a rotating fur brush and an associated vacuum. The fur brush typically rotates at high speeds, and the vacuum induced high air velocity is required to clean the brush and transport the airborne marking particles and other contaminants to a filter.
Examples of selectively positionable roller transfer apparatus constructed to include integral cleaning mechanisms are shown in U.S. Pat. No. 5,101,238 (issued Mar. 31, 1992, in the names of Creveling et al), and U.S. Pat. No. 5,491,544 (issued Feb. 13, 1996, in the names of Kenin et al). While roller transfer apparatus with associated cleaning mechanisms of this type are generally effective in providing for reliable image transfer to receiver members and efficient transfer roller cleaning, under certain circumstances the transfer roller cleaning is insufficient. This is particularly the case when process control patches are developed in the interframe between marking particle images. Contamination is also picked up by the transfer roller from the dielectric support member splice. The cleaning mechanisms described in the aforementioned patents can be ineffective as presently configured to handle such process control patch contamination or dielectric support member splice contamination picked up by the transfer roller. Further, in discharge area development (DAD), the contamination problem may be accentuated (may be material dependent). This is due to the polarity of charge on residual marking particles, or marking particles in the interframe between images, urging the marking particles to the transfer roller to contaminate the roller.
Apparatus and methods for controlling the transfer roller bias to prevent contamination by excess marking particles are known in the art. For example, U.S. Pat. No. 6,014,158 (issued Jan. 11, 2000 in the names of Ziegelmuller et al) shows reversing the polarity of the transfer roller when interframe portions of the dielectric support member pass the transfer roller to substantially prevent attraction of marking particles from process control patches or from the dielectric support member splice. This approach is effective in preventing certain types of contamination. For instance, when negatively charged marking particles are used to develop an image, the transfer roller operates with a positive bias to transfer the image to a receiver. The transfer roller then switches polarity between receivers and the resulting negative bias on the transfer roller repels negatively charged marking particles from process control patches and from the dielectric support member splice. However, some reverse-charged marking particles typically are found among the normally charged particles. For instance, in an apparatus that uses negatively charged marking particles to develop images, a relatively small number of reverse-charged marking particles having a positive polarity may also be present. These positively charged marking particles are attracted to the transfer roller when it is negatively biased, and thereby cause contamination of the transfer roller and, in turn, receiver sheets. Contamination by reverse-charged marking particles is particularly common at the beginning of a reproduction job because reverse-charged marking particles are frequently dislodged from the developing station during start-up, as described more fully below.
It is therefore an object of the present invention to provide an electrostatographic reproduction apparatus and method that provides for a controlled start-up routine that substantially prevents contamination of the image transfer member by both normally charged and reverse-charged marking particles.
In accordance with the present invention, an electrostatographic reproduction apparatus and method are described with an improved start-up process for substantially preventing contamination of the image transfer member by both normally charged and reverse-charged marking particles.
According to one aspect of the present invention, an electrostatographic reproduction apparatus is provided. The reproduction apparatus includes a transfer assembly with an electrically biased transfer roller in nip relation with a dielectric support member for effecting transfer of a pigmented marking particle image from an image area of the dielectric support member to a receiver member. The reproduction apparatus also includes a mechanism for preventing contamination of the transfer roller, including a control for the electrical bias on the transfer roller. The transfer roller bias control includes a power supply generating an electrical output at constant current or constant voltage of a settable polarity. The power supply is connected to the transfer roller for applying an electrical bias of a set polarity to the transfer roller. Means for disengaging the power supply during a start-up phase of reproduction are provided to prevent transfer of residual marking particles from the dielectric support member to the transfer roller.
According to another aspect of the present invention, a method is provided for preventing residual marking particle contamination of a transfer roller in an electrostatographic reproduction apparatus. The reproduction apparatus includes a dielectric support member supporting a pigmented marking particle image, and a power supply for selectively generating an electrical output at constant current or constant voltage of a settable polarity. The power supply is connected to the transfer roller for applying an electrical bias of a set polarity to the transfer roller. The power supply is disabled during a start-up phase of the electrostatographic reproduction apparatus so as to prevent transfer of residual marking particles from the dielectric support member to the transfer roller. The power supply is re-enabled to produce an electrical bias on the transfer roller to transfer the pigmented marking particle image from the dielectric support member to a receiver member.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.